Early influences may accumulate over the life course to impact how well we age. However, relatively little is known about the developmental etiologies of individual differences in age- related cognitive change. Thus, the primary objective of the proposed research is to assess the unique saliency of early childhood factors to adult cognitive maintenance and change versus proximal influences and innovations (genetic and environmental) that emerge across development. We will leverage the strengths of two internationally renowned studies of behavioral development, the Colorado Adoption Project (CAP) and the Longitudinal Twin Study (LTS), each with decades of previously collected cognitive and behavioral data spanning infancy to early adulthood, and conduct a new assessment of 776 adoptive and non-adoptive probands and siblings and 824 twins, ranging in age from 28-38 years. The resulting Colorado Adoption/Twin Study of Lifespan behavioral development and cognitive aging (CATSLife) will be the first prospective longitudinal study of the etiologies of behavioral and cognitive changes during the transition to middle adulthood. The aims are to: conduct a genetically sensitive study of individual differences in behavioral and cognitive change at the cusp of middle adulthood, in 1600 participants studied almost yearly from birth to early adulthood; map individual differences in growth and maintenance of cognitive abilities; evaluate and trace measured physical factors and health behaviors, biochemical markers and measured genetic pathways important to sustaining cognitive performance; and track measured environmental factors that might decrease, sustain or boost cognitive performance. The CATSLife will include expanded assessment of cognitive performance, physical functioning and health behaviors, gene pathways, including measured genetic variation in lipid, synaptic plasticity and cell-signaling paths (based on chip array variants), biochemical markers (e.g., serum lipids), and environmental measures (e.g., engagement in leisure activities). We will assess etiologies of changes in physical health and cognitive functioning, and test whether associations across domains change with age due to changes in genetic variation or self-selection of environments. Measured gene pathway sets, and environmental measures, including engagement in leisure activities and neighborhood-level variables, will be evaluated as potential factors underlying dynamics of genetic variation or environmental selection. Thus, the CATSLife will provide an unparalleled opportunity to assess prospectively the etiologies of cognitive change, and test the saliency of early childhood versus proximal influences on the genesis of cognitive decline.